The great majority of the Internet currently utilizes Internet Protocol Version 4 (IPv4). The address space in IPv4 is becoming exhausted. The eventual exhaustion of the IPv4 address space was recognized some time ago and a common technique of dealing with it was developed. This technique employs network address translation (NAT) wherein a number of private IPv4 addresses, such as within a home or small business network, “hide” behind a single public IPv4 address. Internet Protocol version 6 (IPv6) is a version of the Internet Protocol (IP) that is designed to succeed IPv4.
Note that herein, when the term “IP” is used, it refers generically to IPv4, IPv6, or both types of traffic. When it is desired to refer specifically to IPv4 or IPv6, such specific terminology is employed.
In most residential broadband networks, the connection is “always on” between the user's computer, the user's home gateway internet access router, and the broadband internet service provider. This enables nearly instantaneous connection to sites and services which can be reached via the internet. Typically, each host connected to this network always has an IPv4 address allocated for its exclusive use whenever it is online. However, as IPv4 addresses become scarcer due to the aforementioned exhaustion of the remaining unique IPv4 address space globally, Internet Service Providers (ISPs) are looking for ways to share their current allocations of addresses among multiple customers. The primary way that this is currently accomplished is via Carrier-Grade Network Address Translation (Carrier-grade NAT (CGN), also known as large-scale NAT (LSN)).
CGN is an approach to IPv4 network design in which end sites, in particular residential networks, are configured with private network addresses that are translated to public IPv4 addresses by so-called middlebox network address translator devices embedded in the network operator's network. This permits the sharing of small pools of public addresses among many end sites. This shifts the NAT function and configuration from the customer's premises to the ISP's network. Carrier-grade NAT has also been proposed as an approach for mitigating IPv4 address exhaustion, and easing the transition to IPv6. For further details refer to S. Perreault, Ed., Common requirements for Carrier Grade NATs (CGNs), draft-ietf-behave-lsn-requirements-05, Internet Engineering Task Force, Internet-Draft, Nov. 30, 2011, incorporated by reference herein in its entirety for all purposes.